ABB ETL600, Instruction Manual

Page: 91 / 382

ETL600 1KHW001489-EN
ABB Switzerland Ltd
on the interface module are programmable; if a command requires two
output contacts, for example, both outputs can be assigned to one
interface or even to different interfaces. More inputs / outputs are
easily achieved by inserting additional relay interfaces.
3.6.5.1. Permissive Tripping Schemes
Permissive transfer tripping is the most frequently used scheme for the
protection of transmission lines. The transfer tripping link between the
protection equipment at the ends of the line ensures that all faults can
be cleared in the time of the first zone along 100 % of the line. In a
permissive scheme, the transfer tripping signal from the NSD600 is
connected in series with a local criterion (protection starting, directional
decision or phase selection) and tripping can only take place at the
receiving end, if a transfer tripping signal is being received and the
local protection relay detects a fault in the direction of the protected
line. The reception of a spurious tripping signal caused by interference
on the communications channel cannot therefore give rise on its own
to unwanted tripping. On the other hand, a delayed transfer tripping
signal may mean that a fault on the line is tripped in the time of zone 2
instead of undelayed in zone 1.
High dependability and a short transmission time therefore take priority
over security in a permissive scheme.
The following configuration and settings of the most important
parameters are recommended for
all permissive transfer tripping
schemes
, i.e. underreaching and overreaching distance protection and
directional comparison:
Command application permissive
Command output type: solid state
Command prolongation: 10 ms
The corresponding configuration, setup and further programming
instructions are to be found in section
5.3.3
.
With the above settings, a signal-to-noise ratio of about 3 dB (6 dB
required to fulfill IEC 60834-1, Figure 21) is necessary for successful
transmission of permissive tripping commands (dependability Pmc of
less than 10
-3
) at a transmission time of less or equal than 20 ms. The
rate of unwanted commands for noise bursts of 200 ms duration and
the worst possible signal-to-noise ratio is always less than 10
-6
(security Puc 10 -4 required to fulfill IEC 60834-1, Figure 21).
3.6.5.2. Direct Transfer Tripping
Typical applications for direct transfer tripping are breaker back-up
protection, compensator protection and power transformer protection;
line protection with direct transfer tripping would be an exception. The
requirements with respect to transmission time are generally not too
demanding.
In the case of direct transfer tripping, the tripping command from the
teleprotection equipment goes directly to the circuit-breaker tripping
coil. Thus a spurious tripping signal resulting either from interference
Structure and Function November 2005 3-59